Problems associated with use of a portable radio device within the confines of a vehicle have been known for many years. Primary among these are the shielding effects of the vehicle's metal body on transmission and reception of radio signals.
U.S. Pat. Nos. 5,099,252 and 5,155,494 to Bryant et al show a vehicle-mounted repeater that overcomes many of these problems. The patented system serves as a relay, using a first antenna positioned inside the vehicle to pick up signals transmitted, e.g., from a portable cellular telephone, and then rebroadcasting them through a second antenna positioned outside the vehicle. The system works the other way as well, receiving incoming signals using the outside antenna, and rebroadcasting them for reception by the cellular telephone inside the vehicle by using the inside antenna.
While the Bryant et al invention is particularly illustrated with reference to "passive" operation (i.e. unamplified relay of the radio signals between the inside and outside of the vehicle), the principles thereof are equally applicable to "active" repeater embodiments.
In an "active" repeater, an amplifier circuit is used to increase the strength of a signal received by one antenna before it is rebroadcast by the other. In the context of vehicle mounted active cellphone repeaters, it will be recognized that there are two signals that are being relayed (i.e. the signal transmitted by the telephone, as it is relayed for rebroadcast to the outside antenna; and the incoming signal transmitted by the telephone company, as it is relayed for rebroadcast inside the vehicle). Either or both of these signals can be amplified by an active repeater.
Vehicle-mounted active repeaters are known in the cellular telephone field, as illustrated by the MobilCell product marketed by Decibel Products, a division of the Allen Telecom Group. The MobilCell product includes a plurality of components physically distributed throughout the vehicle. One is the amplifier/repeater, which is housed in a metal box and can be positioned in the car's trunk, under a seat, or under the dash. Associated with the amplifier/repeater is a small "rubber duckie" antenna, which can be mounted directly to the amplifier/repeater unit, or can be connected through an extension cable. (The latter arrangement is apparently used if the amplifier/repeater unit is trunk-mounted, in which case the rubber duckie antenna is mounted somewhere within the passenger compartment, such as near the back seat.) A glass-mounted external antenna is then positioned outside the vehicle and connected to the amplifier/repeater unit (whatever its location) by cable.
This prior art active repeater has a number of drawbacks. One is the cost and complexity associated with having its components distributed throughout the vehicle (e.g. the amplifier/repeater in the trunk, the rubber duckie antenna near the back seat, and the external antenna mounted outside the vehicle). Further, each of these components must be connected to one or more of the other components, such as by cabling and/or connectors, which increases expense and introduces reliability concerns.
The physically distributed nature of the MobilCell system is largely dictated by oscillation concerns. In a more compact arrangement, with the antennas physically closer, the amplifier would likely oscillate. That is, the amplified signal provided by the amplifier/repeater to one antenna for rebroadcast would likely be picked up by the other antenna and fed back into the amplifier input, where it would be further amplified, etc. The amplifier/repeater unit would quickly be overloaded with its own signal and would cease working for its intended operation.
An alternative approach, adopted for example in a product marketed by ORA Electronics, is to eliminate the inside pickup antenna, and instead connect the amplifier unit to the handheld cellular telephone by coaxial cable. Such an approach, however, severely restricts the user's freedom of movement.
In accordance with the present invention, the foregoing and other drawbacks of the prior art are overcome, providing an active repeater that is both simple and reliable. In a preferred embodiment, the amplifier circuitry is disposed in a small housing mounted adjacent the inside of a vehicle window. The inside antenna is coupled to the same assembly. The outside antenna is an on-glass device, mounted on the opposite side of the same window. Oscillation is prevented by the provision of electromagnetic shielding between the inside and outside antennas. In one embodiment, the shielding takes the form of a ground plane on a double-sided circuit board, on which both the inside antenna and amplifier are formed. In another embodiment, the shielding takes the form of the car's metal body, e.g. by positioning the internal and external antennas so the vehicle's metal roof is interposed therebetween.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.